1. Field of the Invention
Embodiments of the present invention relate to an electromechanical transducer element, a liquid discharge head, a liquid droplet discharge device, and an image forming apparatus. Specifically, the embodiments of the present invention relate to the electromechanical transducer element that may be utilized as a piezoelectric element of the liquid discharge head included in an inkjet recording apparatus that is used as an image forming apparatus, or an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, and a multifunction peripheral including these functions; the liquid discharge head including the electromechanical transducer element; the liquid droplet discharge device including the liquid discharge head; and an image forming apparatus including the liquid discharge head or the liquid droplet discharge device.
2. Description of the Related Art
In an image recording apparatus, such as a printer, a facsimile machine, or a copier, or an inkjet recording apparatus that is utilized as an image forming apparatus, an image is formed on an object, such as a sheet that can be a recording medium, by discharging ink droplets from a recording head onto the object. The recording head includes a nozzle that discharges ink droplets; a pressurizing chamber (which is also referred to as an ink flow channel, a pressurizing liquid chamber, a pressure chamber, a discharge chamber, and a liquid chamber) that communicates with the nozzle; and an electromechanical transducer element that applies pressure to ink inside the pressurizing chamber, an electrothermal conversion element such as a heater, or an energy generating unit formed of an oscillation plate that forms a wall surface of an ink flow channel and an electrode facing the oscillation plate. The recording head discharges ink droplets through the nozzle by applying pressure to the ink inside the pressurizing chamber by using energy generated by the energy generating unit.
In general, in the recording head, individual piezoelectric elements are disposed in the corresponding pressurizing chambers so as to generate pressure for discharging ink. The piezoelectric element is referred to as the electromechanical transducer elements, in general. The electromechanical transducer element converts an electrical input into a mechanical deformation. The electromechanical transducer element has a layered structure such that a film formed of, for example, a piezoelectric material is disposed between a pair of an upper electrode and a lower electrode. The pair of the upper electrode and the lower electrode is for providing an electrical input. For a piezoelectric material, for example, a lead zirconate titanate ceramic is utilized (hereinafter, abbreviated as “PZT”). Since such a material includes plural metal oxides as main components, in general it is referred to as a complex metal oxide. There are many technical proposals regarding an electromechanical transducer element including such a piezoelectric material (e.g., Patent Documents 1-6).
Patent Document 1 discloses a technique such that a slope is formed on a side surface of a piezoelectric thin film, and thereby dielectric breakdown is prevented. Patent Document 2 discloses a technique such that an electrical resistivity and a dielectric constant are inclined between a lower electrode and an upper electrode in a film thickness of a piezoelectric material, and thereby the electromechanical transducer element can be driven at a low voltage. Patent Document 3 discloses a technique such that Young's modulus of an oscillation plate and stress of each of thin films are defined, thereby forming a structure that efficiently oscillates.
Patent Document 4 discloses a technique such that a lower electrode is not disposed at end portions in a longitudinal direction of a piezoelectric thin film, and the piezoelectric thin film and an upper electrode are directly formed on an oscillation plate, thereby improving rigidity of the oscillation plate. Patent Document 5 discloses a technique such that a compressive film is formed in an oscillation plate disposed below a piezoelectric thin film, thereby reducing an initial bending of the oscillation plate. Patent Document 6 discloses a technique such that a PZT (lead zirconate titanate) film is pattern-formed by a sol-gel method, thereby providing an inkjet actuator. For the patterning, a PZT precursor film is formed by inkjet printing. Namely, a bank is formed at a desired portion, and a precursor liquid is dropped into the bank. Patent Document 6 discloses that, for forming the bank, a silicone nitride based film is formed, and photolithography/etching is applied.
(A Conventional Method of Forming Individual Piezoelectric Elements)
A conventional method of forming individual piezoelectric elements will be explained below. A film is laminated on a lower electrode by a known film forming technique, such as a vacuum film formation method (e.g., a sputtering method, a MO-CVD method (chemical vapor deposition using a metal organic compound), a vacuum deposition method, and an ion-plating method), a sol-gel method, a hydrothermal synthesis method, an aerosol deposition method (hereinafter, abbreviated as “the AD method”), a coating and thermal deposition method (Metal Organic Deposition (MOD)). Subsequently, an upper electrode is formed, and patterning is performed to the upper electrode by photolithography/etching. Similarly, the patterning is performed to a piezoelectric film and to a lower electrode, and thereby piezoelectric elements are individualized.
A metal complex oxide, especially the PZT is a material to which dry etching is not easily applied. A Si semiconductor device can be easily etch-processed by applying reactive ion etching (RIE). For such a material (metal complex oxides such as the PZT), a special RIE is applied, where ICP plasma, ECR plasma, and helicon plasma are concurrently utilized, so as to increase plasma energy of ion species. However, this can increase the cost of the manufacturing apparatus. Further, it is difficult to improve a selection ratio between the metal complex oxide and a base electrode film. Especially, for a substrate having a large area, unevenness of the etching speed can be a major obstacle for forming a film. The above-described process can be omitted by disposing a PZT film, which is difficult to be etched, at a desired portion. However, such an attempt has not been made, except for few exceptions.
(Cross-Sectional Shape of Conventional Piezoelectric Thin Film)
Next, a cross-sectional shape of a conventional piezoelectric thin film will be explained. In a conventional method of forming the piezoelectric thin film, the piezoelectric thin film is formed over an entirety of a substrate. Subsequently, a required pattern is formed by the dry etching. Therefore, a film thickness of each of the obtained individual piezoelectric thin films is constant over the same substrate. The piezoelectric thin film is required to be efficiently deformed, so that the piezoelectric thin film applies pressure to ink inside a pressurizing chamber, which is formed below the piezoelectric thin film. However, for the conventional piezoelectric thin film, since the film thickness is constant, the deformation of the piezoelectric thin film is suppressed at edge portions of the pressurizing chamber by the piezoelectric thin film itself.
(Conventional Examples of Forming Individual PZT Films)
Next, the hydrothermal synthesis method, the vacuum deposition method, the AD method, and the sol-gel method will be explained as conventional examples of forming the individual PZT films. The hydrothermal synthesis method is a method where the PZT is selectively developed on a Ti metal. When a Ti electrode is patterned, the PZT films grow only on the patterned portions. In order to obtain the PZT film having a sufficient pressure-resistant property by the hydrothermal synthesis method, the film is preferably a relatively thick film having a thickness of greater than or equal to 5 μm (if the film thickness is less than 5 μm, the film is easily damaged by the dielectric breakdown). Therefore, it is difficult to obtain a film having an arbitrary thickness. Further, since the hydrothermal synthesis is performed in a strong alkaline water solution, when the piezoelectric elements are formed on a Si substrate, the Si substrate is protected.
The vacuum deposition method is used for manufacturing an organic electroluminescence display, for example. In this case, a shadow mask is utilized, and for patterning of a luminous layer, the vacuum deposition method is utilized. In the vacuum deposition method, the PZT film is formed while a temperature of a substrate is maintained in a range from 500 degrees Celsius to 600 degrees Celsius. When the metal complex oxide is crystallized, the metal complex oxide exhibits a piezoelectric property. Therefore, in order to obtain the crystallized film, the substrate is maintained to be in the above-described range. A normal shadow mask is formed of a stainless steel. In this case, sufficient masking may not be formed due to a difference between thermal expansion coefficients of the Si substrate and the stainless steel. On the other hand, the possibility of a disposable shadow mask is low. The MO-CVD method and the sputtering method are not suitable for forming individual piezoelectric films due to a large wraparound phenomenon of the deposited films.
As the AD method, a method has been known in which a resist pattern is formed by the photolithography in advance, and the PZT film is formed at a portion where the resist pattern is not formed. Similar to the case of the hydrothermal synthesis method, the AD method is advantageous for forming a thick film. The AD method is not suitable for forming a thin film of less than or equal to 5 μm. The PZT film is also deposited on a resist film. Therefore, a lift-off process is performed after a portion of the deposited film has been removed by a polishing process. Since a uniform polishing process of a large area is complicated and the resist film does not have a heat-resisting property, a film is formed by the AD method at room temperature, and the formed film is converted into a film which exhibits the piezoelectric property through a post-annealing treatment.
As the sol-gel method, Patent Document 6 proposes an example of a method in which the PZT precursor solution is applied by the inkjet method (liquid droplet discharge method). However, since a viscosity of the PZT precursor solution that has been applied by the inkjet method is low, the PZT precursor solution spreads on platinum, which is to be a substrate. Further, when a large amount of PZT precursor solution is applied at once, an edge at an outer peripheral portion of a pattern rises and a central portion becomes thin. Therefore, the sol-gel method is not suitable for obtaining a thick film.
Non-Patent Document 1 discloses a technique related to forming a thin film of a metal complex oxide by the sol-gel method. Further, Non-Patent Document 2 discloses that a self assembled monolayer (SAM) film of alkanethiol can be formed on a film of Au. Non-Patent Document 2 discloses that the SAM pattern is transferred by the micro-contact printing method in which this phenomenon is used, and the SAM pattern is utilized for a subsequent process, such as an etching process.    Patent Document 1: Japanese Registered Patent No. 4117504    Patent Document 2: Japanese Patent Laid-Open Application No. 2008-147682    Patent Document 3: Japanese Registered Patent No. 3725390    Patent Document 4: Japanese Registered Patent No. 3636301    Patent Document 5: Japanese Registered Patent No. 3019845    Patent Document 6: Japanese Patent Laid-Open Application No. 2011-108836    Non-Patent Document 1: K. D. Budd, S. K. Dey, D. A. Payne, Proc. Brit. Ceram. Soc. 36, 107 (1985).    Non-Patent Document 2: A. Kumar and G. M. Whitesides, Appl. Phys. Lett., 63, 2002 (1993).